The invention relates generally to the field of fluid dynamics, and in particular to the dampening or stabilizing of pressure spikes or pulses within a fluid. More specifically, the invention relates to the dampening or stabilizing of pressure peaks or pulses that are created by a reciprocating pump.
The use of pumps to move fluids is well known. Typical reciprocating pumps draw fluid into a pump cylinder where the fluid is pressurized and discharged to force the pressurized fluid through a pipe. When pressurizing the fluids in this manner, various operational issues should be addressed both on the suction side of the pump and on the discharge side of the pump.
Attention to the suction side of the pump is important because if the fluid is unable to properly fill the pump cylinder, the pump cannot operate smoothly. Hence, the pump will be unable to smoothly move the fluid through the discharge system. Moreover, improper feed conditions can result in damaging pressure pulsations within the fluid and cause early failure of valves, valve seats, springs, plungers, expendable parts, and the like.
To operate efficiently, the fluid should stay in continuous contact with the piston or plunger. Factors which prevent such contact include: (1) the action of the pump itself and the demands this action places on the feed system; (2) the nature of the fluid being pumped; and (3) the suction head requirements.
A reciprocating piston pump has a piston that is reciprocated back and forth as the result of crank rotation. Although the crank rotates at a constant rate of rotation, the piston is translated at different rates. For example, a ten degree movement of the crank at the end of the stroke moves the piston only about one percent of the stroke. At midstroke, a ten degree crank movement moves the piston about eight percent of the stroke. Finally, at each end of the stroke, there is no piston movement.
On the intake stroke, the piston begins moving toward the crankshaft at a rapidly increasing speed until it reaches midstroke. This is followed by a rapidly decreasing speed that culminates at the end of the stroke when the piston movement stops momentarily before beginning the discharge stroke. Under even the best of controlled conditions, the changes in fluid velocity that result from the inherent changes in piston velocity can impose severe demands on the pump's feed system.
For the pump cylinder to fill completely, the column of fluid must first be placed in motion at a rate which maintains continuous contact with the pump piston. Additionally, during the intake stroke, the pressure in the cylinder must be greater than the vapor pressure of the fluid being pumped to prevent gas formation and incomplete filling of the cylinder.
The head required on the suction side of the pump is the pressure required at the suction manifold to completely fill the cylinder when the piston is on the suction stroke. Hence, the suction head must be sufficient to: (1) overcome any frictional losses through the piping and fittings; (2) overcome the weight and spring tension of the valves; (3) maintain the fluid pressure above its vapor pressure; and (4) accelerate the flow of fluid in the suction system. Of these four requirements, the acceleration head (item 4) is probably the most commonly overlooked, yet causes more piping and pumping problems than the other three items.
In a suction feed system, the fluid has a mass or weight. The weight of the fluid is determined by the length and diameter of the suction line and specific gravity or density of the fluid.
Because of its mass, every fluid possesses a resistance to flow. Acceleration head is the pressure required to overcome the effect of inertia and to accelerate the fluid as the pump's suction demands. This acceleration head is a function of the fluid mass in the suction line, the pump speed, the number of plungers, and the pump displacement. At higher rotational speeds of many present day pumps, or with longer length suction lines, the acceleration head should be taken into account in designing a piping system.
If the feed system has a high acceleration head that has not been compensated for, the fluid in the suction piping system is unable to accelerate as rapidly as the plunger or piston. This results in loss of contact between the fluid and the piston, creating a vacuum in the cylinder itself. This occurs just prior to midstroke at maximum piston velocity, and has been termed "midstroke cavitation effect." When the piston breaks contact with the fluid, the feed system pressure into the pump decreases producing high frequency pulses in the fluid.
As the piston decelerates after passing midstroke, the fluid rushes into the cylinder producing a high pressure on the piston face. At the end of the stroke, the piston reverses direction and absorbs the kinetic energy of the fluid column. The resulting pressure reversals cause abnormal valve action and are transmitted throughout the pump and its power unit. These pulsations exist regardless of whether the feed system is pressurized to a high level.
Hence, one object of the present invention is to utilize a pulsation dampener or stabilizer to address the effects created by the acceleration head and pump action. As such, it would be desirable to provide a pulsation stabilizer or dampener which is placed on the suction side of the pump to compensate for the acceleration head of the feed system to insure that the cylinder is completely filled at all times, thereby maintaining continuous contact between the fluid and the piston.
On the discharge side of the pump, severe pressure pulsations may be caused by the crank piston arrangement of the pump, the piping, and poor suction conditions. When multiple pump installations are discharged into a common header, the high pressure pulsation surges from each pump may overlap into the common discharge system.
Hence, another object of the invention is to provide a pulsation stabilizer or dampener to stabilize or dampen the pressures on the discharge side of the pump to enable the system to be more safe, dependable, and efficient.
Another important issue that should be addressed on both the suction side of the pump and the discharge side of the pump, is that the pumping system may be operated over a wide range of line pressures. Hence, it is still a further object of the invention to provide a pulsation stabilizer or dampener which is effective at various line pressures. In this way, a single stabilizer or dampener may be employed to dampen or stabilize a wide range of pressure pulses.